I. Field of the Invention
The invention is directed to an improved method of recovering viscous oil from a subterranean oil formation separated into at least two horizontal zones by an intervening horizontal layer having lower vertical permeability than the oil formation.
II. Description of the Prior Art
Many oil reservoirs, such as heavy oil or tar sand formations, exist which contain vast quantities of oil which cannot be recovered by conventional techniques because the oil is so viscous that it is substantially immobile at reservoir conditions. Therefore, some form of supplemental oil recovery must be used in such formations to decrease the viscosity of the oil sufficiently to allow it to flow through the formation to the production well and then be brought to the surface of the earth. Thermal recovery techniques which decrease the viscosity of such oil and are therefore suitable for stimulating the recovery thereof include steam flooding and in-situ combustion.
Steam has been utilized in the past for thermal stimulation of viscous oil in so-called steam drive or steam throughput processes in which steam is injected into the formation on a substantially continuous basis through an injection well, and oil, having reduced viscosity, is recovered from the formation from a spaced-apart production well. The mechanism of the oil production by steam flooding is believed to involve the condensation of the steam upon contact with the cooler formation sands and the migration of the resulting hot water through the viscous oil, thereby reducing the viscosity of the oil and allowing it to flow more easily. This oil is then produced from production wells spaced-apart from the injection wells. In prior art, steam flooding has been applied to viscous oil reservoirs separated into at least one lower and one upper zone by at least one intervening horizontal layer of a different material, e.g., shale, with much lower vertical permeability than the rest of the formation. It was thought that such a horizontal layer formed a complete barrier to the flow of steam. Accordingly, each of the zones of the formation was treated separately with steam by injecting the steam separately into each of the zones and producing oil from each zone independently. Such a manner of operating the steam flooding process often resulted in a substantially delayed steam breakthrough as compared to steam flooding operations in viscous oil reservoirs forming substantially one vertically extending reservoir without an intermediate horizontal layer. Therefore, the commencement of the increased oil production which accompanies steam breakthrough from such layered reservoirs was also delayed.
Accordingly, a need still exists in the art for providing an improved method of recovering viscous oil by steam flooding of a layered reservoir.
This and other objects of the invention will become apparent to those skilled in the art from the following description thereof.